The present invention relates to the construction of ponds containing bodies of liquid, and particularly to the bottom construction of such ponds. The invention is especially useful in the construction of thermal ponds, particularly non-convective solar ponds, to prevent heat losses by seepage through the bottom, and therefore the invention is described below with respect to this application, but it will be appreciated that it could also advantageously be used in other types of thermal ponds, such as liquid storage ponds, to prevent liquid losses by seepage through the bottom.
Generally speaking, there are two types of solar ponds, namely solar evaporation ponds in which salt solutions are concentrated and/or salts are precipitated by the utilization of solar energy for the evaporation of the solvent water; and non-convection solar ponds, e.g. as described in Israel Pat. No. 12561 and U.S. Pat. No. 3,372,691, in which a vertically-extending salt concentration gradient is produced in the pond to produce a vertically-extending temperature gradient to enable the pond to act as a solar energy collector in which the concentrated solution at the bottom of the pond is heated to a high temperature, e.g. up to 100.degree. C. and over.
One of the difficulties in the construction of solar ponds is the requirement for impermeability of the bottom of the pond. This requirement is considerably more stringent, and more difficult to fulfill, in non-convective solar ponds than in ordinary solar evaporation ponds or other thermal ponds, mainly because the bottom layer of the solution in a non-convective solar pond is considerably hotter, and therefore any seepage results in a loss of both concentrated solution and of energy. In addition, non-convective solar ponds use solutes which may be expensive salts whose loss by seepage substantially increases the cost in operating the pond.
Another difficulty experienced in non-convective solar ponds, particularly when the bottom is constructed of compacted earth, is the evolution of gases from below the bottom into the pond. This may occur when the bottom temperature reaches about 60.degree. C., and is therefore particularly troublesome in non-convective solar ponds frequently operated at bottom temperatures of 100.degree. C. and higher. This evolution of gases is probably caused by the disintegration of some components in the soil, the escape of air dissolved in ground water, and/or the expansion of gases in the soil. In any event, the evolution of these gases into the bottom of the pond tends to destroy the concentration gradient in the pond and to foul its water such that the pond may have to be abandoned.
In order to overcome the above difficulties, plastic covers (e.g., polyethylene, polyvinyl chloride), such as are used to seal water reservoirs and ponds, have been considered for use in the bottom of non-convective solar ponds, but these plastic covers deteriorate rapidly at the operating temperatures (up to 100.degree. C. and above) at the bottom of the pond. Accordingly, expensive heat resistant materials (such as special heat-resistant plastic and metal sheets) have been used, but such materials are extremely expensive; moreover, even when they are used, it has been found that excessive seepage may still occur as a result of corrosion (if metal), aging, and imperfect joining of the plastic or metal sheets.